Tissue expanders are devices that are implanted beneath the skin and then gradually inflated to stretch the overlying tissue. Such expanders are used to create a pocket for receiving a permanent prosthesis and/or to generate increased skin surface area so that skin can be utilized for grafting or reconstruction.
In the case of mammary implants, tissue expanders are used to create the mammary pocket that will ultimately receive the permanent mammary implant. These expanders are commonly formed of a silicone polymer shell. After implantation, saline or some other fluid is periodically injected into the expander over time, for example through an injection port, until the desired sized pocket is achieved.
With known mammary tissue expanders, as the inflation process continues, resistive pressure from the tissue on the anterior side of the expander can cause the expander to expand in undesired directions (i.e., axially and laterally). In order to minimize the undesired expansion, most surgeons select a smaller expander than needed and overinflate the expander to 200-300% of the rated volume of the expander. This allows the surgeon to utilize the smaller starting foot print of a smaller expander to accommodate for the undesired axial and lateral expansion. Overinflating a smaller expander is undesirable for various reasons. Although expanders are technically tested up to twice their nominal inflation volume, 200-300% inflation could reduce the safety margin of the device. Further, since the footprint is small, at 200-300% inflation the inflated shape is not anatomically correct, but rather is more round or ball-like, which could lead to rotation or flipping over of the implant within the tissue pocket.
Thus, is would be desirable to provide an expandable mammary implant that better provides for the appropriate directional tissue expansion for any given size.